This relates to electronic devices with clocks and other radio-frequency interference sources, and more particularly, to ways in which to align interference sources with receiver filters to reduce undesired radio-frequency interference.
Electronic devices are often provided with wireless communications capabilities. For example, electronic devices may contain cellular telephone network transceiver circuitry for supporting long-range wireless links with cellular network base stations. Electronic devices may also use short-range wireless communications links. For example, electronic devices may communicate in wireless local area network bands at 2.4 GHz and 5 GHz (e.g., using IEEE 802.11 standards) and may communicate using Bluetooth® links at 2.4 GHz.
Devices that operate in wireless communications bands include radio-frequency transceiver circuits. These circuits, which are sometimes referred to as radios, may be used to handle transmitted and received signals in one or more radio-frequency bands of interest.
Electronic devices also have other components such as displays, processors, and memory. Clock circuits are used to distribute a common time reference to these components. For example, a crystal oscillator may be used to generate a reference clock signal. Clock circuitry may be used to create clock signals such as square waves from the output of the crystal oscillator. For example, a phase-locked loop circuit may be used to create a clock signal at a multiple of the crystal oscillator's frequency.
A clock that operates at a given frequency f may produce signals at harmonic frequencies (e.g., fundamental harmonic f and higher order harmonics 2 f, 3 f, 4 f, 5 f, etc.). In a given electronic device, these harmonic frequencies may overlap with the frequencies of other signals in the device such as the frequencies used by radio-frequency transceiver circuitry. If care is not taken to properly isolate these overlapping signals, the device may not operate properly.
As a result of the potential for undesirable signal collisions, extensive consideration is given to proper electromagnetic shielding in modern electronic devices. This typically entails providing additional electronic components in a device whose purpose is to reduce the impact of signal collisions. For example, certain components may be electromagnetically shielded by mounting the components within conductive enclosures. Signal interference can also be minimized by using filter networks.
These schemes generally help to reduce signal collisions between clock sources and component operating frequencies. Nevertheless, there can be penalties associated with shielding schemes. Metal enclosures consume valuable space and add cost and complexity to a device. Particularly in small-form-factor devices, there may be insufficient space for a conductive enclosure. Filtering components may add undesirable cost to a design and must be carefully selected to avoid adversely affecting device reliability.
It would therefore be desirable to provide ways in which to reduce the adverse impact of potential signal collisions in electronic devices with wireless communications circuitry.